Really?! Shouldn't come lose that easily. All digital cameras are that way and I've never had one come lose in a bump, even when I dropped one so hard it broke. I did fail to put my card in my DC500 correctly once, and got maybe 4 shots with onboard memory.
Well I can not say for certain, only that it has happened a few times, I test it when I first put it all together then I get into the water and the card is not present. then On the surface I reset the card and all is well. I can only assume the card is getting bumped or else there is some other sort of random problem.